Bearings typically support a rotating or otherwise moving article, such as a journal or roll. In high temperature applications, bearings often comprise a refractory metal, ceramic or composite. In such applications, bearings may even be in direct contact with molten metals, such as molten zinc or aluminum.
Applications include galvanization, which is the process of forming a protective, anti-oxidant zinc layer on a base metal. A continuous galvanizing apparatus comprises a bath of molten zinc with a sink roll at least partially immersed in the bath. The bath is maintained at a temperature sufficient to keep the zinc molten. A housing at each longitudinal end of the sink roll include at least one refractory bearing. The refractory bearing includes a wear surface that supports the sink roll. The sink roll forces the base metal, which is often in the form of a sheet or wire, into the molten zinc. The sink roll rotates against the wear surface of the refractory bearing as the base metal passes into the molten zinc, under the sink roll, and finally out of the molten zinc.
The requirements for refractory bearings can be severe. Molten zinc is at least 420° C., typically around 460° C., and corrodes many common bearing materials. Mechanical abrasion is a persistent complication. The bearings can wear quickly and must be replaced frequently. Replacement requires the shutdown of the galvanizing operation while new bearings are inserted. Disruption of a continuous galvanizing operation results in significant operator costs and lost production.
Prior art bearings include metal housings, often in the shape of a ring. During operation, the roll contacts a working face of the refractory bearing. The high temperature and corrosive environment destroy metal bearings relatively quickly and cause the sink roll to rotate eccentrically, thereby reducing galvanizing efficiency. Worn bearings must be replaced, often at great cost. Prior art also includes metal housings having inserts comprising a refractory material selected for its erosion and corrosion resistance. The roll contacts the inserts instead of the metal housing. The inserts are substantially more resistant to wear and corrosion than the metal housing alone and can extend the life of bearings many times. Typically, the inserts are polygonal shapes and are embedded in a plurality of cavities along the working face of the metal housing.
At room temperature, the inserts are secured tightly into the cavities. This may be accomplished using a retaining plate and one or more wedges to improve the tightness of fit. The retaining plate can be welded to the housing and may extend at least partially over the insert in the cavity. Still, inserts have a tendency to fall out at operating temperatures because the thermal expansion of the metal housing is greater than the ceramic inserts. Loss of an insert causes the journal to wobble or otherwise rotate eccentrically. Fortunately, the journals pressing against the inserts can hold the inserts in the cavities despite thermal expansion; however, pressure can be lost when the galvanizing operation is stopped or slowed. In such situations, the journal may separate from the insert sufficiently to permit the insert to fall from its cavity. The tendency of a ceramic insert to loosen and fall from a metal cavity increases with temperature and would be even more likely at higher temperatures, such as with molten aluminum baths which are typically at least around 700° C. and more commonly around 715° C.
A need persists for a refractory bearing comprising a housing and a wear-resistant insert where the insert is fixedly secured to the housing without the need for a retaining plate. Advantageously, the bearing would be easily manufactured of substantially inexpensive materials and would be suitable for use with molten zinc and aluminum.